The TARDIS seems to have returned the Doctor and Sarah Jane to modern-day
England, but it quickly becomes apparent that something is very wrong:
the people behave oddly, the calendar has just one day on it, coins are
all minted from the same date. The time travellers soon realise that they
are not on Earth at all, but on a simulacrum created by the Kraals, who
are using the replicated village to help them prepare for their imminent
invasion of Earth.

Production

Although Terry Nation had written or cowritten ten Doctor Who
stories since 1963, only Season One's The Keys Of
Marinus omitted his most famous creations, the Daleks. Latterly,
Nation had had an informal agreement with the Doctor Who production
team to provide one Dalek serial per season, the most recent of these
being Genesis Of The Daleks. When it came
time to discuss Nation's contribution to Season Thirteen, however, he was
made aware that producer Philip Hinchcliffe and script editor Robert
Holmes were far less interested in spotlighting old monsters than their
predecessors had been.

Instead, Nation began work on two non-Dalek storylines. The first,
entitled “The Enemy Within”, was commissioned on November
29th, 1974, while the other, called “Return To Sukannan”, was
requested on February 13th, 1975. The second idea appears to have been
abandoned, but on February 27th, Nation was contracted to provide full
scripts for “The Enemy Within”, now under the title “The
Kraals”.

Originally, a key plot element was that the Kraal androids
were mirror images of the people they were imitating

Originally, a key plot element was that the Kraal androids were in fact
mirror images of the people they were imitating, and this was how the
Doctor deduced that “Sarah Jane” was actually a robot in
episode two. It was eventually decided that this would be too technically
demanding to realise, and the action was suitably amended. Corporal Adams
was conceived as a regular army private, and the scripts indicated that
his first name was Richard. As well, Nation envisaged the Kraals as
somewhat insectoid in appearance, although this idea was not used in the
design work.

Around the start of July, the adventure's title reverted to “The
Enemy Within”; it was classified as Serial 4J. At about the same
time, it was learned that Nicholas Courtney would not be available to
reprise his role as Brigadier Lethbridge Stewart in the story, because of
obligations to a theatrical tour. With the start of production looming,
Holmes replaced Lethbridge Stewart with a new character called Faraday, to
be played by Patrick Newell. Faraday was conceived as a brigadier, and was
later demoted to colonel. On July 14th, the serial's title was finalised
as The Android Invasion.

A familiar face was assigned to direct Serial 4J: Hinchcliffe's
predecessor as producer, Barry Letts. Letts had been lined up to oversee a
biography of Marie Curie, but when this became entangled in internal BBC
politics, Letts was given work as a script doctor on various drama
programmes. On January 21st, he had been commissioned to write a storyline
for Doctor Who entitled “The Prisoner Of Time”, but
nothing came of this. Finally, he secured permission to serve out the
remainder of his BBC contract in a directorial capacity, and took the
reins of The Android Invasion. Letts' last such credit on Doctor
Who had been for the final Third Doctor adventure, Planet Of The Spiders, at the end of Season
Eleven.

Letts began work on The Android Invasion with five days on location
in Oxfordshire. July 21st was spent at the National Radiological
Protection Board in Harwell, whose grounds served as those of the Defence
Station. The next two days were devoted to scenes in the woods around the
false Devesham. The principal location was Tubney Wood in Tubney, although
a small amount of work on the 22nd took place at Worsham Quarry in Witney.
It was at Tubney Wood that Tom Baker performed a sequence in which he was
submerged in a river. Unfortunately, Baker swallowed too much water and
had to be taken to a hospital to have his stomach pumped. Finally,
material set in “Devesham” itself was filmed in the village
of East Hagbourne on July 24th and 25th.

Making their last Doctor Who
appearances, neither Ian Marter nor John Levene enjoyed recording The Android Invasion

Rejoining Doctor Who for the studio recordings were Ian Marter and
John Levene. Marter had just completed his regular engagement as Harry
Sullivan in Doctor Who at the end of the previous production block,
and was making his lone return to the programme. For Levene, The
Android Invasion marked the last of a long series of regular
appearances as Benton dating back to 1968. Levene left the acting
profession in 1977 and took up a number of different jobs: cruise line
entertainer, head of an audio-visual company, even private detective. More
recently, Levene moved to the United States and began acting again,
including a role in the horror film CanniBallistic! (under his real
name, John Anthony Blake). Neither Marter nor Levene found The Android
Invasion a pleasant experience, as the new production team's
deemphasis of the UNIT format had become progressively apparent. Levene
also missed Courtney's presence on set.

The first studio session for The Android Invasion took place on
Monday, August 11th and Tuesday the 12th, in BBC Television Centre Studio
3. These days were chiefly concerned with taping the majority of episodes
one and two, respectively, although part three scenes in the Kraal cell
and corridor were recorded on the 11th, and sequences in the
Disorientation Chamber and corridor from the same installment were
completed on the 12th.

The second block was scheduled for exactly two weeks later, on August 25th
and 26th. This time, the venue was TC8. Again, Letts primarily
concentrated on one episode each day, but also performed some shooting for
other installments. Part two scenes in Styggron's control room and the
corridor of the Defence Centre were taped on the 25th. On the 26th,
material in the scanner room from part one and in the loading bay from
part three was also recorded. Unfortunately, Letts ran out of time on
this day, resulting in the loss of a key scene from late in the final
installment which would have explained how the Doctor reactivated his
android duplicate, as well as accounting for the Kraal invasion
armada.